In parenteral nutrition or blood transfusion systems, the liquid supplied from a bottle is injected into a patient by means of a pump. Air may be introduced into the circuit of liquid, for example if the pump continues to work when the bottle is empty, or through a leak in the parenteral perfusion or transfusion tube due to the exerted pumping forces. Because of the danger that they represent for the patient, the air bubbles must be detected.
The use of an air bubble detector, incorporated in the perfusion or transfusion tube, is impossible from a practical and economical standpoint. Indeed, the tube must be sterilizable as well as readily dismountable and re-assembled; in addition, the detector must not transmit any electrical current to the patient's body via the liquid which may be very conducting; finally, the tube being a consumer good, its cost must remain as low as possible.
It is therefore important to use air bubble detectors which are external to the perfusion or transfusion line, which makes the detection rather difficult, all the more so that the liquids can be of very varied types with completely different characteristics.
Among the known air bubble detectors, some are of optical type with a photo-emitter and a photo-receptor, disposed in facing relationship on two opposite sides of a conduit in transparent material carrying the liquid. The passage of an air bubble causes a variation in the transparency of the medium situated between the photo-emitter and the photo-receptor, hence a variation which can be detected from a signal supplied by the photo-detector.
Use of these known detectors raised difficulties due to the very wide dispersion of the transparency of the liquids normally carried in the conduit; transparent, translucid or opalescent liquids.
In order to overcome these difficulties, it has been proposed to use air bubble detectors of optical type in which the reference value, as a function of which the variation of the signal produced by the photo-detector is analyzed, is pre-adjusted manually. But, the detector adjusted in this way, is only capable of operating correctly inside a limited range of liquid transparency. Moreover, an error of adjustment may always occur, with serious consequences.
It has also been proposed in French patent application No. 2 361 644, to use a plurality of optical emitter-receiver pairs, one of which is used to make an automatic adjustment or pre-conditioning of the detector, making a distinction between transparent liquid and opaque liquid. The fact remains that, for every system of operation ("transparent" liquid or "opaque" liquid) the range of transparency remains wide, which always makes it difficult to select a reference value. Moreover, this automatic adjustment can be performed at the cost of a rather great complexity of the detector.